The present invention relates to a method and device for analyzing trace components present in a gas with high sensitivity and high accuracy by means of spectroscopic analysis using a diode laser as the light source. More particularly, the present invention is designed to facilitate optimization of the conditions for measurement.
A spectroscopic analysis method in which the degree of light absorption by a gas is measured using a diode laser as the light source is widely used as a method for analyzing trace impurities in gas, since it offers good measurement accuracy and sensitivity.
FIG. 10 is a schematic structural diagram showing an example of a conventional gas spectroscopic analysis device. In this device, the laser light oscillated from diode laser 11, the light source, is collimated at collimating lens system 12, and then split into three lines, i.e., first through third lines, by two beam splitters 13,13. The laser light in the first line is projected on measuring gas-cell 14. The intensity of the outgoing transmitted light which has passed through measuring gas-cell 14 is then detected at first light detector 15. The laser light in the second line is projected on reference gas-cell 16. The intensity of the outgoing transmitted light which has passed through reference gas-cell 16 is then detected at second light detector 17. The intensity of the laser light in the third line is detected at third light detector 18.
A sample gas supply system 23 is provided to measuring gas-cell 14, by means of which the sample gas is introduced into cell 14 at a suitable reduced pressure and constant flow rate. The impurities to be measured that are included in the sample gas are supplied to reference gas-cell 16, and the absorption peaks due to these impurities are detected.
An InGaAsP, InGaAs, GaInAsSb, GaInSbP, AlInSb, AlInAs, AlGaSb or the like may be suitably employed for diode laser 11. Diode laser 11 is not limited to these however. In addition, a tunable diode laser which can oscillate laser light of a wavelength suitable for analysis may be used.
A device having a sensitivity to the oscillation wavelength band of diode laser 11, the light source, is employed for the first through third light detectors 15, 17, 18. A light sensor such as a Ge photo diode may be employed, for example. The respective outputs from these first through third light detectors 15, 17, 18 undergo signal processing at first through third lock-in amplifiers 19,20,21, are relayed to computer 22, and then subject to data processing as necessary.
A temperature controller 24 for controlling the temperature of the laser element, an LD driver 25 for supplying current to and driving laser 11, and a function generator 26 for serving as a frequency modulating device for modulating the oscillation frequency of laser 11 based on a frequency modulation method, are provided to diode laser 11. Temperature controller 24, LD driver 25, and function generator 26 are connected to computer 22. By adjusting the temperature of the laser element using temperature controller 24, the oscillation wavelength of laser 11 is changed to approach the central wavelength of the absorption peak for the impurities being measured, after which the temperature of the laser element is maintained at a constant value. In addition, by continuously changing the injection current (direct current component) to laser 11, the oscillation wavelength of laser 11 is continuously changed. In addition, by introducing a modulation signal (alternating current component) to LD driver 25 that is based on the frequency modulating method from function generator 26, and superimposing this modulation signal (alternating current component) on the injection current (direct current component) to laser 11, frequency modulation can be applied directly to the laser light oscillated from laser 11.
The phrase xe2x80x9coscillation wavelength of laser 11xe2x80x9d as used in this specification means the wavelength which is not in a frequency modulated state, i.e., the central wavelength.
In this example, frequency modulation is applied to the laser light, and only the twice component of the modulated frequency is extracted using first through third lock-in amplifiers 19, 20, and 21. Specific data processing is then performed by computer 22 to obtain the second derivative spectrum. It is known that good measurement sensitivity can be obtained by this method (Japanese Patent Application, First Publication No. Hei 5-99845). In addition, it is known that the peak intensity of the second derivative spectrum that is obtained can be increased by placing the sample gas in a reduced pressure state (International Publication Number WO 95/26497).
In the aforementioned frequency modulating method, the current i introduced into the diode laser can be expressed as
i=I0+axc2x7sin(xcfx89t)
Here, I0 is the direct current component, axc2x7sin(xcfx89t) is the alternating current component (modulation signal). xcex1 is the modulation amplitude (amplitude of the modulation signal), and xcfx89 is the modulation angular frequency. As a result of this type of frequency modulation, the frequency (wavelength) of the laser light varies cyclically at a fixed amplitude around the central frequency (central wavelength) when there is no modulation. The amplitude by which the frequency (wavelength) of the laser light varies becomes greater as modulation amplitude xcex1 becomes bigger. The cycle by which the frequency (wavelength) varies is determined based on the frequency of the modulation signal (modulation frequency).
If the modulation amplitude of the laser light is made large in the measurement, then the spectrum width becomes bigger. However, the variation in output power also becomes greater, so that there is an increase in noise as a result.
FIG. 11 shows an example of the second derivative spectrum obtained using a gas spectroscopic analysis method employing this type of device. In this figure, the oscillation wavelength is shown on the horizontal axis, while the second derivative value (optional units) of the light absorption intensity is shown on the vertical axis. The average of the respective differences between peak value P in the second derivative spectrum and minimum values A and B on the left and right hand of peak value P, i.e., differences ISL and ISR(ISL=Pxe2x88x92A, ISR=Pxe2x88x92B), is the absorption intensity (absorption intensity =(ISL+ISR)/2). The ratio of the absorption intensity and the standard deviation of the background noise (indicated by n in the figure) is the S/N ratio. The symbol Win the figure indicates the wavelength interval between the minimum values on the left and right hand of the peak.
The modulation amplitude of the laser light and the measurement pressure effect the S/N ratio in this type of spectroscopic analysis method. Accordingly, it is necessary to optimize these conditions in order to perform highly sensitive measurements. In order to optimize the modulation amplitude and measurement pressure, it has been the practice to employ a method in which a highly pure base gas and a sample gas containing a trace component which is to be measured in the base gas are each measured while gradually varying the measurement pressure and the modulation amplitude, and the modulation amplitude and measurement pressure at which the S/N ratio is maximal are obtained. The optimization of the modulation amplitude and the measurement pressure must be carried out each time there is a change in the sample gas. Accordingly, this requires much effort and time, and has been a cause of increased cost.
The present invention was conceived in view of the above-described circumstances, and is intended to facilitate the optimization of measurement conditions in a method for analyzing trace impurities in a sample gas by obtaining the second derivative spectrum of the light absorption intensity by passing frequency modulated diode laser light through a reduced pressure gas which is the target of measurement.
In order to resolve the above-stated problems, the present invention""s gas spectroscopic-analysis device is provided with a tunable diode laser; a frequency modulating device for performing frequency modulation of the diode laser; a device for passing laser light oscillated by the diode laser through the sample gas; a device for measuring the intensity of the laser light which has passed through the sample gas; and a device for obtaining the second derivative spectrum from the measured laser light intensity; wherein this gas spectroscopic analysis device is provided with a modulation amplitude calculating device for calculating the optimal value of the modulation amplitude from the diode laser characteristics and controlling the frequency modulating device so that the modulation amplitude of the laser light oscillated from the diode laser becomes an optimal value. By employing a device of this design, optimization of the modulation amplitude of the laser light can be easily carried out. Further, since a second derivative spectrum which has a good S/N ratio is obtained, it is possible to perform highly sensitive measurements.
Further, by providing a spectrum calculating device for calculating the peak absorption intensity and the wavelength interval between the minimum values on the left and right hand of the peak in the second derivative spectrum, and a pressure adjusting device for adjusting the pressure of the sample gas based on the results calculated by the spectrum calculating device, optimization of the measurement pressure can be carried out easily, and highly sensitive measurements can be performed.
In the present invention""s gas spectroscopic analysis method, laser light which has been frequency modulated from a tunable diode laser is oscillated, the laser light is passed through the sample gas, the intensity of the transmitted light is detected, and the second derivative spectrum is obtained using the detected light intensity, the method being characterized in that the optimal value of the modulation amplitude of the laser light is set so that the wavelength interval between the minimum values on the left and right hand of the peak in the second derivative spectrum is 0.0116 nm. By setting the wavelength interval between the minimum values on the left and right hand of the peak in the second derivative spectrum to be 0.0116 nm, the S/N ratio can be maximized irrespective of the type of sample gas.
The wavelength interval between the minimum values on the left and right hand of the peak in the second derivative spectrum can be set to 0.0116 nm by setting the value of the modulation amplitude of the laser light to the same value as the injection current necessary to change the oscillation wavelength of the diode laser by 0.0232 nm only.
Having set the modulation amplitude of the laser light to an optimal value, the optimal value of the pressure of the sample gas is set so that the absorption intensity of the second derivative spectrum is maximized. As a result, a good S/N ratio and absorption intensity can be obtained. Accordingly, the number of times measurements of the sample gas must be repeated in order to optimize the measurement pressure can be reduced as compared to the conventional art, and the optimization of the measurement pressure, which required much time and effort in the conventional art, can be carried out simply and quickly.
Further, the calibration curve for the trace impurities in the sample gas is formed in a state such that the modulation amplitude of the laser light and pressure of the sample gas have each been set to optimal values. As a result, a calibration curve measured under optimal conditions can be obtained, and trace impurities in the gas can be measured with high sensitivity and accuracy.